Query filter syntax in MongoDB

Introduction

Query filters help you find only the data you want from a big collection. They act like a search tool to pick matching items.

When you want to find all users older than 25 in a user list.

When you need to get all orders with status 'shipped'.

When you want to find products priced below $50.

When you want to search for documents containing a specific word.

When you want to filter records by date or category.

Syntax

MongoDB

{ field: value }

The filter is a JSON object where keys are field names and values are what you want to match.

You can use operators like $gt (greater than), $lt (less than), $eq (equals) inside the filter.

Examples

Find documents where the age field is exactly 30.

MongoDB

{ age: 30 }

Find documents where the price is less than 50.

MongoDB

{ price: { $lt: 50 } }

Find documents where status is either 'shipped' or 'delivered'.

MongoDB

{ status: { $in: ["shipped", "delivered"] } }

Find documents where the name starts with the letter 'A'.

MongoDB

{ name: { $regex: "^A" } }

Sample Program

This query finds all orders where the status field is exactly 'shipped'.

MongoDB

db.orders.find({ status: "shipped" })

Important Notes

Filters are case-sensitive by default.

You can combine multiple conditions using $and, $or operators.

If you want to match a field that does not exist, use $exists operator.

Summary

Query filters let you pick only the data you want by specifying conditions.

Filters use JSON objects with field names and values or operators.

You can use many operators to create flexible and powerful searches.

Practice

1. What does a MongoDB query filter do?
{ age: 25 }

easy

A. Deletes documents where age is 25

B. Updates documents to set age to 25

C. Selects documents where the age field is exactly 25

D. Creates a new document with age 25

Solution

1. Step 1: Understand the role of a query filter

   A query filter in MongoDB is used to find documents that match certain conditions, not to modify or delete them.

2. Step 2: Analyze the filter { age: 25 }

   This filter matches documents where the field age has the value 25 exactly.

3. Final Answer:

   Selects documents where the age field is exactly 25 -> Option C

4. Quick Check:

   Query filter = select matching documents [OK]

Hint: Filters pick matching data, not modify or delete [OK]

Common Mistakes:

• Confusing filter with update or delete commands
• Thinking filter creates or changes documents
• Assuming filter matches partial or range values without operators

2. Which of these is the correct syntax to find documents where score is greater than 80?

easy

A. { score > 80 }

B. { score: gt 80 }

C. { $score: { gt: 80 } }

D. { score: { $gt: 80 } }

Solution

1. Step 1: Recall MongoDB operator syntax

   MongoDB uses JSON objects with operators starting with a dollar sign, like $gt for 'greater than'.

2. Step 2: Check each option

   { score: { $gt: 80 } } uses { score: { $gt: 80 } } which is correct syntax. Others miss the dollar sign or use invalid syntax.

3. Final Answer:

   { score: { $gt: 80 } } -> Option D

4. Quick Check:

   Use $gt for greater than in filters [OK]

Hint: Operators start with $ inside field object [OK]

Common Mistakes:

• Omitting $ before operator
• Using comparison operators like > directly
• Wrong operator names without $

3. Given the collection documents:

{ name: "Alice", age: 30 }
{ name: "Bob", age: 25 }
{ name: "Carol", age: 30 }

What will this query return?
db.collection.find({ age: 30 })

medium

A. [{ name: "Alice", age: 30 }, { name: "Carol", age: 30 }]

B. [{ name: "Bob", age: 25 }]

C. [] (empty array)

D. Syntax error

Solution

1. Step 1: Understand the filter condition

   The filter { age: 30 } matches documents where the age field equals 30.

2. Step 2: Identify matching documents

   Documents for Alice and Carol have age 30, so both match. Bob has age 25, so does not match.

3. Final Answer:

   [{ name: "Alice", age: 30 }, { name: "Carol", age: 30 }] -> Option A

4. Quick Check:

   Filter matches exact age 30 documents [OK]

Hint: Filter matches exact field values without operators [OK]

Common Mistakes:

• Expecting only one document returned
• Confusing filter with update or delete
• Thinking filter matches partial or range without operators

4. What is wrong with this MongoDB query filter?
db.collection.find({ age: $gt: 20 })

medium

A. Using $gt instead of $gte

B. Missing curly braces around $gt operator

C. Field name should be $age, not age

D. No error, query is correct

Solution

1. Step 1: Check operator syntax in filter

   MongoDB requires operators like $gt to be inside an object as a value for the field key.

2. Step 2: Identify the syntax error

   The query uses { age: $gt: 20 } which is invalid. It should be { age: { $gt: 20 } } with curly braces around $gt: 20.

3. Final Answer:

   Missing curly braces around $gt operator -> Option B

4. Quick Check:

   Operators must be inside braces as field values [OK]

Hint: Put operators inside braces after field name [OK]

Common Mistakes:

• Omitting braces around operator
• Using wrong operator names
• Misplacing $ sign

5. You want to find documents where status is either "active" or "pending". Which filter correctly does this?

hard

A. { status: { $in: ["active", "pending"] } }

B. { status: { $or: ["active", "pending"] } }

C. { $or: { status: "active", status: "pending" } }

D. { status: "active" || "pending" }

Solution

1. Step 1: Understand how to match multiple values

   MongoDB uses $in operator to match a field against any value in a list.

2. Step 2: Analyze each option

   { status: { $in: ["active", "pending"] } } uses { status: { $in: ["active", "pending"] } } which is correct. { status: { $or: ["active", "pending"] } } uses invalid operator $or inside field. { $or: { status: "active", status: "pending" } } has wrong syntax for $or. { status: "active" || "pending" } uses invalid JavaScript syntax.

3. Final Answer:

   { status: { $in: ["active", "pending"] } } -> Option A

4. Quick Check:

   Use $in for matching any value in list [OK]

Hint: Use $in with array for multiple value matches [OK]

Common Mistakes:

• Using $or inside field instead of top-level
• Wrong syntax for $or operator
• Using JavaScript operators inside filter